1. Department of Statistics, University of Michigan, 323 West Hall, 1085 South University, Ann Arbor, Michigan 48109, U.S.A. yqhe@umich.edu
2. Department of Statistics and Finance, University of Science and Technology of China, Anhui, 230026, China mb0529@mail.ustc.edu.cn zzh98052@mail.ustc.edu.cn
3. Department of Statistics, University of Michigan, 323 West Hall, 1085 South University, Ann Arbor, Michigan 48109, U.S.A. gongjun@umich.edu